1. Field of the Invention
The present invention relates to a sprocket, and more particularly to a multistage sprocket mechanism for a bicycle.
2. Description of the Prior Art
Typical multistage sprocket mechanisms are mounted on a crank or a rear hub of a bicycle and comprise a number of sprockets for engaging with a driving chain and spaced from each other at a predetermined interval. In order to facilitate the shifting of the driving chain from one of the sprockets to the other sprocket, various kinds of ideas have been developed to improve the same. The prior multistage sprocket mechanisms comprise: U.S. Pat. No. 536,813 to Macphail et al.; U.S. Pat. No. 619,537 to Bufford; U.S. Pat. No. 1,583,221 to Carlson; U.S. Pat. No. 3,478,614 to Shimano; U.S. Pat. No. 3,498,148 to Gerbasi et al.; U.S. Pat. No. 3,756,091 to Miller; U.S. Pat. No. 3,772,932 to Nagano; U.S. Pat. No. 3,956,943 to Yamasaki; U.S. Pat. No. 4,018,095 to Shimano; U.S. Pat. No. 4,268,259 to Segawa et al.; U.S. Pat. No. 4,330,286 to Nagano; U.S. Pat. No. 4,521,207 to Husted; U.S. Pat. No. 4,773,893 to Su et al.; U.S. Pat. No. 4,8113,916 to Valin; U.S. Pat. No. 4,889,521 to Nagano; and U.S. Pat. No. 5,133,695 to Kobayashi. Some of the prior arts comprise a number of teeth having recesses formed in the side portions for engaging with the chains so as to facilitating the shifting of the driving chain. In U.S. Pat. No. 4,889,521 to Nagano, the sprockets each includes one side having a number of recesses formed thereon and extended beyond the teeth so as to form chain guide portions for engaging with the driving chain and for facilitating the engagement of the driving chain with the sprockets and for preventing the driving chain from engaging with the other sprockets. However, after the driving chain has been shifted from one sprocket to the other sprocket, the driving chain may will be engaged with the previous sprocket.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional multistage sprocket mechanisms.